1. Field of the Invention
The present invention relates to a piezoelectric ink-jet printhead. More particularly, the present invention relates to a piezoelectric actuator for generating a driving force to eject ink from a piezoelectric ink-jet printhead and a method of forming the same.
2. Description of the Related Art
Generally, an ink-jet printhead is a device that ejects small volume ink droplets at desired positions on a recording medium, thereby printing a desired color image. Ink-jet printheads are generally categorized into two types depending on which ink ejection mechanism is used. A first type is a thermal ink-jet printhead, in which ink is heated to form ink bubbles and the expansive force of the bubbles causes ink droplets to be ejected. A second type is a piezoelectric ink-jet printhead, in which a piezoelectric crystal is deformed to exert pressure on ink causing ink droplets to be ejected.
FIG. 1A illustrates a plan view of a conventional piezoelectric ink-jet printhead. FIG. 1B illustrates a vertical cross-sectional view taken along line I-I′ of FIG. 1A.
Referring to FIGS. 1A and 1B, a flow path plate 10 having ink flow paths including a manifold 13, a plurality of restrictors 12, and a plurality of pressurizing chambers 11 is formed. A nozzle plate 20 having a plurality of nozzles 22 at positions corresponding to the respective pressurizing chambers 11 is formed on a lower side of the flow path plate 10. A piezoelectric actuator 40 is disposed on an upper side of the flow path plate 10. The manifold 13 is a common passage through which ink from an ink reservoir (not shown) is introduced into each of the plurality of pressurizing chambers 11. Each of the plurality of restrictors 12 is an individual passage through which ink from the manifold 13 is introduced into a respective pressurizing chamber 11. Each of the plurality of pressurizing chambers 11 is filled with ink to be ejected and collectively they may be arranged at one or both sides of the manifold 13. Volumes of each of the plurality of pressurizing chambers 11 change according to the driving of the piezoelectric actuator 40, thereby generating a change of pressure to perform ink ejection or introduction. To generate this change in pressure, an upper wall of each pressurizing chamber 11 of the flow path plate 10 serves as a vibrating plate 14 that can be deformed by the piezoelectric actuator 40.
The piezoelectric actuator 40 includes a lower electrode 41, piezoelectric layers 42, and upper electrodes 43, which are sequentially stacked on the flow path plate 10. A silicon oxide layer 31 is formed as an insulating film between the lower electrode 41 and the flow path plate 10. The lower electrode 41 is formed on the entire surface of the silicon oxide layer 31 and serves as a common electrode. The piezoelectric layers 42 are formed on the lower electrode 41 and are positioned at an upper side of each of the respective pressurizing chambers 11. The upper electrodes 43 are formed on the piezoelectric layers 42 and serve as driving electrodes for applying a voltage to the piezoelectric layers 42.
To apply a driving voltage to the above-described piezoelectric actuator 40, a flexible printed circuit (FPC) 50 for voltage application is connected to the upper electrodes 43. More specifically, wires 51 of the flexible printed circuit 50 are disposed on the upper electrodes 43 and then are heated and pressurized to bond the wires 51 to upper surfaces of the upper electrodes 43.
Referring to FIG. 1A, the pressurizing chambers 11 have a narrow and elongated shape. Accordingly, the piezoelectric layers 42 and the upper electrodes 43 similarly have a narrow and elongated shape. In view of this configuration, to firmly bond the wires 51 of the flexible printed circuit 50 to the upper electrodes 43, portions of the upper electrodes 43 to be bonded to the wires 51 must be sufficiently long. For example, in a conventional ink-jet printhead, lengths of the upper electrodes 42 are about twice as long as lengths of the pressurizing chambers 11.
Even though the piezoelectric layers 42 may have the same length as the pressurizing chambers 11, it is required that they have a greater length than the upper electrodes 43 to insulate the upper electrodes 43 and the lower electrode 41 and to support the upper electrodes 43. Resultantly, areas of the piezoelectric layers 42 are unnecessarily and disadvantageously increased. When the areas of the piezoelectric layers 42 are increased, a capacitance increases. Therefore, a load increases during driving the piezoelectric actuator 40 and a response speed of the piezoelectric actuator 40 decreases.
The upper electrodes 43 are generally formed by coating a conductive metal paste to a predetermined thickness onto upper surfaces of the piezoelectric layers 42 by screen printing followed by sintering. For this reason, the upper electrodes 43 have rough and coarse surfaces. Accordingly, even though a binding length between the upper electrodes 43 and the flexible printed circuit 50 may be sufficiently long, as described above, a binding force therebetween may be insufficient. As a result, there is a high likelihood that the upper electrodes 43 and the flexible printed circuit 50 may become separated when the actuator 40 is driven for a long time.